Production of groundwood pulp



April 26, 1960 1.. R. NESTOR EI'AL 2,934,279

PRODUCTION OF GROUNDWOOD PULP Filed June 29, 1955 AME/W025 [id/S4420 27/1 5702 19055274 15/2465 Unte PRODUTION F GROUNDWOOD PULP Application .lnne 29, 1955, Serial No. 518,818

2 Ciaims. (Cl. 241-200) This invention relates to a novel method for the production of groundwood pulp; to apparatus and equipment employed therein; and to the novel pulp and paper products produced.

Groundwood pulp provides a major source of newsprint and other large-volume and low-cost paper products, and is also employed in significant quantities in the manufacture of high grade book and magazine papers, where it improves the opacity and printability of the product. Such pulp is ordinarily produced by grinding green, seasoned, or pretreated logs or billets against a revolving pulpstone having an appropriately grooved or dressed abrading face. In a typical pulpstone grinder such as the Great Northern 2-pocket magazine type grinder, the stone employed is 54 inches across the face and 62 inches in diameter. Billets cut to a standard length of approximately 48 inches, i.e., slightly shorter than the width of the stone, and held in two pockets arranged approximately opposite each other and covering about the circumference of the grinder, are pressed against the face of the revolving stone, with the longitudinal axis of the billet parallel to the axis of rotation of the stone. The fibers of the wood are torn from the log to form a groundwood pulp which, after screening to remove shives and chunky or coarse fragments, and mixing with minor amounts of chemical pulp and other desired additives, is ready for the mixed stock chest. The groundwood pulp produced is high in non-fibrous fines and coarse fiber bundles. Paper produced with significant proportions of such pulp is necessarily rough surfaced and weak.

An important object of this invention is therefore to provide novel and useful groundwood pulp having fibrous components of high length-to-diameter ratio and which is applicable to the production of usefully strong, thin and opaque paper, including high quality book and magazine paper as well as lower cost products such as newsprint or insulating-board. The pulp fibers are more supple and have better conformability than prior groundwood fibers. Another object is to provide means for making improved groundwood pulps from the less used species of woods, e.g., the hardwoods. A further object is to provide groundwood pulp which blends well with chemical pulp and provides a homogeneous furnish. Other objects of the invention include the provision of a novel method of grinding wood, capable of directly providing such groundwood pulp, and the provision of novel apparatus and equipment applicable for accomplishing such results. The production of novel and useful paper products and other sheet materials employing the resulting fibrous base stock is a further object of the invention. A particular object is the preparation, on a commercial scale, of groundwood pulp having fibrous constituents of increased length-to-diameter ratio and capable of forming test sheets which in the dry state are much higher in opacity than are the comparable test sheets formed from groundwood pulp made by methods and with apparatus previously available. Another important object is to provide means for rapidly and easily converting the pulp-grinding operation to produce a different type of pulp having different desired characteristics.

Groundwood pulp having the novel and highly useful qualities hereinabove noted is prepared on a commercially significant scale, in accordance with the principles of the present invention, by employing flexible grinding structures as herein described and under conditions of fully longitudinal wet grinding, wherein the movement of the grinding surface at the wet surface of the properly conditioned log or billet is continuously substantially parallel to the longitudinal direction of the fibrous components naturally occurring in the wood.

The invention will now be further described in connection with the accompanying drawing, in which:

Figure 1 is a cross-sectional elevation illustrating one embodiment of apparatus suitable for longitudinal grinding of pulpwood in accordance with the principles of this invention;

Figure 2 is a section of a surface-coated flexible grinding device as employed in connection with the apparatus of Figure 1;

Figure 3 is a cross-sectional elevation of a modification of a portion of the apparatus of Figure 1; and

Figures 4 and 5 are photomicrographs of comparable significant fractions of typical groundwood pulps produced respectively by transverse grinding on a conventional pulpstone grinder and by true longitudinal grinding in accordance with the principles of the present invention.

The stated objects of the invention as well as other objects and advantages are attained by proceeding as indicated in Figure 1, wherein a surface-coated belt 101 is driven past the surface of the logs or billets 105 while being backed up by a stationary flat-surfaced back-up plate 104. The active grinding surface is thereby maintained substantially planar, with each separate particle traveling across the effective grinding area in substantially a straight line. The logs are contained within a hopper, the sides of which, not shown in the drawing, are parallel to the edges of the abrasive belt 101. The billets are forced against the moving grinding surface by pressure, indicated by the arrow 107, applied through a pressure plate 106, which as shown is substantially parallel to the surface of the back-up plate 104 but which may be held at any desired small positive or negative angle with said back-up plate by means of the quadrant assembly 108. A thrust plate 109 supports the billets against the downward thrust of the grinding-belt 101 and prevents the passage of any but very small unground,

segments of wood into the storage sump 116. The coated flexible belt 101 is carried by drums 102 and 103, the required driving power being transmitted to the belt through drum 102. Water under pressure is sprayed against the belt from nozzles 110 and 111, to remove the fibers from the belt surface and to keep the belt and the surface of the wood cool and adequately wet for proper grinding. Additional quantities of water may be added during the grinding operation, for example through nozzle 117 and longitudinally between the billets. A cabinet encloses the entire assembly. The wet pulp accumulates in the storage sump 116 and is screened through screen 114, any unground segments of Wood being removed through the clean-out opening 112, and the pulp product being routed to storage or to further processing equipment through outlet valve 113.

Green wood produces a product of maximum strength and is generally preferred. Dry wood must be well soaked prior to grinding, and the resulting pulp produces a sheet which is not as strong as that from green wood, but the sheet is higher in opacity. The wood may be otherwise pretreated where desired. For example, the billets may be steamed or subjected to chemical action, e.g., pretreatment with sodium sulfite solution, prior to grinding.

The pulp produced with fine-grit grinding coatings and with the grain of the wood exactly parallel to the line of travel of the grit particles has the largest proportion of long slender fibers and fibrous fines, any deviation from such parallelism resulting in coarser fibers and increased proportion of non-fibrous fines The character of the raw material makes true parallelism generally impossible. However, with the sides of the hopper parallel to the edges of the grinding-belt and the billets placed lengthwise of the hopper, as indicated in Figure I, the lateral angle will generally average within a very few degrees of zero and in any event, it has been found that slight variation in the lateral angle is not noticeably harmful to pulp quality.

Placing the billets crosswise of the hopper will be seen to result in transverse grinding, in which case, and employing grit of appropriate size and shape, a pulp is produced which is very similar to that produced on conventional pulpstone grinders, being high in non-fibrous fines and having a high proportion of coarse chunky fiber bindles. The process employing the surface-coated grinding-belt has the advantages, as compared to the conventional pulp stone method, of providing for ready interchangeability of grinding surface as different types of pulp are desired or diiferent types 'of wood are provided, and also of immediately providing an open, cavitated grinding-surface and thus avoiding variability in pulp properties ordinarily occasioned during conditioning of a pulpstone. Transverse grinding may obviously be carried out with the flexible coated grinding sheet applied as a cover over a conventional pulpstone or equivalent drum-shaped wheel, or as a flexible belt backed up by a rotating wheel rather than a fiat platen, while maintaining the same advantages of interchangeability and constancy of grinding-surface. With the quadrant assembly 108 of Figure 1 set so that the face-plate 106 is parallel to the abrasive belt, the angle of attack during longitudinal grinding operations will be, on the average, very close to zero. It has been found that excellent pulp is obtained with the grain of the wood at zero angle of attack or at a very small positive angle of attack, whereas a negative angle of attack produces a much finer fiber and may result in burning and in degradation of pulp properties. Hence it is found desirable to fix the pressure plate 106 at a slight positive angle so that the average angle of attack of the fibrous axis of the wood will be zero or slightly positive. At higher positive angles a coarser pulp is obtained. Consequently an angle of attack of very close to zero degrees is preferred for the production of pulp designed for high grade book papers, whereas a moderate positive angle of attack is preferred in grinding pulp for insulating-board or felted products. Most pulps are ground at settings of from about minus 2 to plus 5 degrees, and in no case will the angle of attack be more than about 5 degrees in the negative direction or about degrees in the positive direction in producing the longitudinally ground pulp products of this invention.

A positive angle of attack is here defined as an angle between billet and grinding surface at which the distance between the grinding surface and the fiber axis of the wood increases in the direction of travel of the abrasive surface.

The character of the fiber is controlled to a considerable extent by the force applied on the pressure plate 106 of Figure l, and this pressure must therefore be rather carefully controlled. In a typical experiment employing billets 6 inches long in a hopper 6 inches in width, i.e., with a hopper area of 36 sq. in., and with a grinding-belt sparsely coated with No. 120 size grit and running at 4200 linear feet per minute, and operating as shown in Figure 1 with soaked green aspen wood, a total pressure of about 500-650 pounds, equivalent to about 14-18 pounds per square inchof projected area, was found to produce a quality of pulp suitable for the production of book grade paper. In another experiment, southern pine was longitudinally ground with a grindingsurface coated with No. 80 size grit under a pressure of 17-23 lbs. per sq. in. of projected area to produce a pulp of newsprint grade which yielded paper of increased rigidity. In all such operations, reducing the pressure produces much finer fibers, in some cases so fine as to be of insufiicient strength, and the rate of fiber production is greatly decreased. At higher pressures, coarser fibers are produced and the degree of fibrillation is greatly decreased, but the rate of pulp production is increased. Low pressures are useful where relatively coarse and widely spaced grits are used.

The photomicrograph of Figure 4 illustrates, at a magnification of 75 diameters, a typical portion of the 200- mesh fraction of groundwood pulp as produced by transverse grinding on a conventional dressed pulpstone. The pulp contains some relatively slender fibers but has a large proportion of coarse chunky fiber bundles. Figure 5 illustrates a comparable sample of the ZOO-mesh fraction of pulp made by longitudinal rather than transverse grinding and demonstrates advantages to be gained from the novel pulp-making procedures herein described and illustrated. It will be seen from Figure 5 that most if not all of the fibers and fiber bundles have a much higher length-to-diameter ratio than in the case of the transversely ground material. Test sheets prepared from whole pulp exemplified by the sample of which a portion is illustrated in Figure 5 are surprisingly dense and opaque when compared with sheets of the same basis weight made with groundwood pulps obtained by previously known processes and exemplified by the pulp from which the fraction illustrated in Figure 4 was obtained. For example, a sheet of paper having a basis weight of 37 lbs. per 25 x 38/500 ream, made from the longitudinal pulp, was equal in opacity to a comparable sheet made from the transverse pulp at a basis weight of 43 lbs. per ream. In another test, sheets made from whole pulp of the type exemplified by Figure 5 had a wet tensile strength substantially three times that of sheets made to the same basis weight from Whole pulp of the type exemplified by Figure 4.

Wet tensile strength is conveniently determined by procedures described by Walter Brecht and Karlheinz Klemm in an article entitled A Procedure for the Rapid Determination of the Quality of Groundwood in Groundwood Manufacture, published in Das Papier, volume 5, pages 489-503 (1951).

The apparatus employed is described by Waiter Brecht in an article entitled The Measurement of the Wet Strength of Paper, also appearing in Das Papier, volume 1, page 127 (1947). Tests made on the No. 814 Darmstadt type instrument there described, employing pulp strips produced from longitudinally ground poplar and operating at 22% moisture, provided instrument readings of 178-191. Conventional transverse grinding of the same Wood gave pulp strips which, tested at 20% moisture, provided a typical instrument reading of 72.

' 'Allowing for the' slight difference in test conditions, it

'will be seen that the transversely ground material has fiber on the dry basis, and compared for density. The longitudinal pulp produced sheets of significantly greater density, as shown in the following table:

Efiect of pulp formation on density of dry sheet 6 drums 302 and 303, of which drum 302 is driven from a source of power, not shown, is drawn over a supporting belt 305 carried on drums 306 and 307. Drum 306 In a further example, green aspen billets were longitudinally ground in accordance with the procedures here-' in described, and the resulting whole pulp was formed into test sheets and compared for opacity with sheets made in the same way from pulp prepared from the same lot of wood by conventional transverse grinding on a stone. Opacity was determined in terms of the lightscattering coeflicient of the test sheets in accordance with standard practice as set forth in TAPPI procedure T-425 m, corrected August 1944. This scattering coefficient is reported in units of reams per pound, a ream being 500 sheets 25 in. x 38 in. Assuming the white light reflectance of the two pulps to be identical, then the ratio of the basis weights of sheets having identical opacity will be identical with the ratio of the scattering 'coefiicients. The highest value obtainable with trans- 'verse grinding was 0.125 whereas longitudinal grinding resulted in a value of 0.180, and hence the basis weights of sheets having identical opacity will be in the ratio "of .180/.125 when comparing transversely ground pulp with longitudinally ground pulp. Thus the opacity of a 50 lb. sheet made from transverse pulp would be the same as that of a sheet made from longitudinal pulp to a basis weight of but 35 lbs. The longitudinal pulp was readily miscible with sulfite pulp and other chemical pulps, yielding a homogeneous pulp mixture having excellent paper-forming characteristics, whereas the transverse pulp in such mixtures yielded products having less desirable properties in a number of respects.

The amount of pressure which it is possible to apply under any given set of circumstances in pressing the wood against the grinding-surface is determined by the tensile strength of the backing of the flexible belt 101 of Figure l. Frictional forces set up between the back surface of the belt and the adjacent surface of the backup plate 104 are extremely high, and it is difficult to provide adequate and effective lubrication between these surfaces. The friction developed results in the formation of much heat, which must be dissipated, thus adding to the problem. The length of the logs or billets 105 and the total pressure applied against the logs, must therefore be restricted below a point at which failure of the grinding-belt might occur. Some assistance is obtained by the introduction of large quantities of water,

e."g., through nozzle 117, or preferably between the back of the belt and the face of the back-up plate through small holes or pores in the plate. Other and more effeetive lubricants may be substituted. Further assistance may be obtained through the use of grinding-belt structures of maximum tensile strength and water resistance. However the simplified system illustrated in Figure l is ordinarily restricted to use with logs or billets of somewhat less strength than those ordinarily used in transverse grinding operations.

It will be apparent that the apparatus of Figures 1 and 3 may be operated equally well in a horizontal rather than a vertical position, where appropriate alterations in positioning of the screens, housings, etc, are made.

An alternative apparatus is indicated in Figure 3. In

is driven at a surface speed equal to that of drum 302. A back-up plate 304, having a flat surface, is located in back of the two belts 305 and 301 at an area beside the hopper assembly holding the logs or bolts 308. The logs are pressed against the flat grinding-surface adjacent the back-up plate 304 by a force 310 applied through a pressure plate 309 and are prevented from being removed from the hopper by a thrust plate 311. The angular relationship between the log and the grindingsurface is regulated as in the apparatus of Figure 1. The pulp is removed from the surface of the belt, screened, and collected, also in accordance with the principles outlined in connection with Figure 1.

Where the backing of the flexible belt 101 of Figure l is inadequate in tensile strength to resist both the grinding action against the log and the frictional action against a flat stationary back-up plate, the introduction of a back-up belt 305, as in Figure 3, provides a means of overcoming the frictional force and of permitting the successful use of the grinding-belt in the large-scale production of groundwood pulp. The supporting belt 305 may itself be of fabric construction, or alternatively may be a flexible metal belt.

The grinding-belt and back-up belt of Figure 3 may also advantageously be combined into a single element by adhesively binding the loose grit, or a preformed gritcoated sheet material, to a flexible metal belt or series of connected flat metal plates and employing the resulting belt structure in an apparatus such as that of Figure 1. Alternatively, a grit-surfaced belt structure may be provided by converting the initially smooth planar surface of the metal belt or metal plates to a cut-ting surface, as by etching or knurling, followed if desired by case-hardening of the etched or knurled surface. With such a structure, and with the assistance of auxiliary supplies of water applied as previously indicated along the grinding surface, it is possible to grind logs of substantially increased length and under substantially increased pressure for more economical and efficient production of groundwood pulp.

Another modification of Figure 3 involves the utilization of a chain structure having interlocking links, in place of the combination of belt 305 and back-up plate 304. Such a chain is capable of maintaining a flat surface under high pressure applied from one direction while being entirely flexible under pressure applied in the opposite direction. It will be obvious that the flat outer surface of the chain, formed from the closely fitting outer surfaces of the separate links thereof, must be sufi'iciently flat and smooth to provide an adequate bearing surface to support the grinding-belt under the pressure applied to the logs or billets being ground.

Details of a preferred type of grinding-belt structure are given in Figure 2, in which a backing element 201 carries a series of round-edged grits 204 firmly bonded to the backing by means of a binder adhesive 202 and a sizing adhesive 203. As illustrated, the grit particles have a blocky shape characteristic, and have dull rounded outer exposed edges as indicated at 205. Also as illustrated, the grinding surface is open-coated; that is, the grit particles are separated, in this case, by a distance '7 approximately equal to the diameter of the individual grains. Furthermore a substantial portion, at least about one-third, of the total surface of each particle is exposed. Greater or lesser openness of coating or separation of such grains may be employed depending, among other things, on the linear speed of the grinding-belt during the grinding process. In general the projected area of the effective surfaces of the grains should account for at least about of the total projected area of the surface. Minimum speed requires that the grits be placed quite qualities.

' pulp having improved printability and opacity; and the closely together, with a minimum of intervening spaces.

At extremely high belt speeds the spacing between grits may be substantially increased. The effective surfaces of the grains should lie in substantially the same plane. As above noted, the exposed edges205 of the grains or grits 2% are rounded or dull, in this respect differing sharply from the abrasive particles employed on conventional abrasive belts. As with such abrasive belts, the grit particles here employed may be silicon carbide, aluminum oxide, garnet, corundum, or other particulate material of suitable hardness. In the grinding-belts of the present invention these particles are desirably of blocky rather than slivery shape characteristics and have rounded or dull edges rather than sharp edges.

The desired grain shape is obtained by tumbling the sharp grains, produced by crushing of the massive mineral starting material, in the presence of stones or balls having a mild polishing action and for a time sufiicient to remove or dull the sharp edges without cracking the individual grains. Anotherprocedure involves reducing the massive material to desired grain size with a hurricane mill operated at a high degree of recirculation, any initially formed sharp edges thereby being effectively dulled. The grinding edges of normally sharp-edged abrasive grains have also been dulled or rounded to an effective degree by first bonding the grains to a flexible backing as in forming an abrasive belt, and then driving the coated surface over a wooden block or other suitable pressureapplying base kept covered with a slurry of much finer abrasive particles in a liquid carrier such as water. This last-mentioned procedure has the further advantage of removing any excess of adhesive bond from the exposed surfaces of the grinding-particles and thereby improving the fibrillation of the pulp produced with such structures.

Any surface irregularities in the grit layer are desirably avoided, so that the projecting rounded grit surfaces are in substantially the same plane with respect to the back surface of the flexible belt. This result may be accomplished by grinding an irregularly surfaced coated sheet so as to remove the excessively protruding points, followed by abrading with a slurry of fine abrasive particles as above described. Another method, known as transfer-coating or inverse coating, involves locating the prepolished grains against a planar surface in a temporary binder, bonding the free exposed surfaces of the grains to the desired permanent flexible backing, and then stripping the sheet from the planar surface and removing the temporary binder.

It has been found that grinding-belt structures such as just described effectively and efliciently reduce the wood to groundwood pulp in which the fiber bundles have a remarkably high ratio of length to diameter, combined with a maximum of fibrillation. Belts coated with spherical granules have likewise provided wellfibrillated long-fibered pulp but at a considerable increase in power consumption. On the other hand, typical abrasive belts, coated with sharp abrasive particles, produce coarse or chunky fiber bundles with low length-to-diameter ratio and with little if any fibrillation. Although such belts might be operated at reduced power cost per ton of pulp as compared to those carrying dull abrasive grain, the fibrous product would be of little value for paper-making.

The character of the pulp, and the rate at which it is produced, is also influenced by the grit size of the abrasive grains forming the grinding surface. Very small method also makes possible substantial increases in the proportion of aspen to spruce and in the proportion of groundwood to chemical pulp which can safely be used in making such products. Grits -150 have been used in making book and magazine pulp, from which paper having improved opacity and strength per unit weight is produced. The whole pulp produces test sheets which are more dense and more opaque than comparative sheets produced from the best available transversely-ground pulp.

In all cases the pulp contains a high proportion of fibrous components having a high length-to-diameter ratio and containing fibrous fines. The equipment required is much less massive and cumbersome than that formerly required. The individual grinding units are easily and quickly convertible from one type of grinding to another, simply by changing the flexible grindingbelt. The pulp characteristics, e.g., fiber classification and freeness values, are readily controlled as desired. A complete change in pulp characteristics may be secured by merely substituting a grinding-belt having different size particles, different particle distribution, or different particle shape; by changing the pressure at which the wood is forced against the grinding surface; or by changing the speed of travel of the belt. The flexibility of the system is thus greatly improved over that of previously known pulp-grinding systems.

What is claimed is as follows:

1. Apparatus for the preparation of fibrous pulp from wood by controlled longitudinal grinding, comprising in combination: an open-faced grit-coated grinding-belt;

pulleys for supporting and driving said belt; a fiat back-up plate, contacting the grit-free side of a flat portion of the belt; a hopper adjacent the grit-coated side of the belt opposite said back-up plate, and including sides parallel to the direction of advance of said belt, a thrust plate for preventing loss of substantial sections of unground Wood from the hopper during grinding, and a pressure plate for forcing wood billets in said hopper against said grinding-belt, said pressure plate being adjustable as to angle of attack; spray nozzles for directing water at high velocity against said grinding-belt to remove ground wood clinging thereto and to maintain the grinding area in wet condition; and means for collecting the ground wood product.

2. Apparatus for the preparation of fibrous pulp from Wood by controlled longitudinal grinding, comprising in combination: a flexible incompressible water-resistant grinding-belt having a dull planar flat grinding-surface; pulleys for supporting and driving said belt; a fiat backup plate contacting the reverse surface of a fiat portion of the belt; a hopper adjacent the grinding-surface of the 'belt opposite said back-up plate, and including means for holding elongate wood billets substantially parallel to the direction of advance of said belt, thrust plate means for preventing loss of substantial sections of unground wood from the hopper during grinding, and adjustable pressure plate means for forcing wood billets in said hopper against said grinding-belt at a desired low angle of attack; means for forcing water at high velocity against said grinding-belt to remove ground wood clinging thereto and to maintain the grinding area in Wet condition; and means for collecting the ground wood product.

(References on following page) 10 References Cited in the file of this patent 1,405,177 Quenehen et a1. I an. 31, 1922 2,332,329 Maca Oct. 19, 1943 UNITED STATES PATENTS 2,336,798 Nash Dec. 14, 1943 253,654 Allen Feb. 14, 1882 2,575,656 Coe Nov. 20, 1951 3 9,992 Goodwin Sept. 28, 1886 5 2,642,359 Scott June 16, 1953 860,367 Gramelspacher July 16, 1907 947,491 Bein Jan 25, 1910 FOREIGN PATENTS 1,344,180 McMillan June 22, 1920 49,67 Germany "Nov-25,1889 

1. APPARATUS FOR THE PREPARATION OF FIBROUS PULP FROM WOOD BY CONTROLLED LONGITUDINAL GRINDING, COMPRISING IN COMBINATION: AN OPEN-FACED GRIT-COATED GRINDING-BELT, PULLEYS FOR SUPPORTING AND DRIVING SAID BELT, A FLAT BACK-UP PLATE CONTACTISNG THE GRIT-FREE SIDE OF A FLAT PORTION OF THE BELT, A HOPPER ADJACENT THE GRIT-COATED SIDE OF THE BELT OPPOSITE SAID BACK-UP PLATE, AND INCLUDING SIDES PARALLEL TO THE DIRECTION OF ADVANCE OF SAID BELT, A THRUST PLATE FOR PREVENTING LOSS OF SUBSTANTIAL SECTIONS OF UNGROUND WOOD FROM THE HOPPER DURING GRINDING, AND A PRESSURE PLATE FOR FORCING WOOD BILLETS IN SAID HOPPER AGAINST SAID GRINDING-BELT, SAID PRESSURE PLATE BEING ADJUSTABLE AS TO ANGLE OF ATTACK, SPRAY NOZZLES FOR DIRECTING WATER AT HIGH VELOCITY AGAINST SAID GRINDING-BELT TO REMOVE GROUND 